Dual-walled drill pipe used for drilling bore holes in earth formations are well known. Generally, bore holes are formed by rotating or percussively-rotating a drill string, which may be up to several hundred feet in length, into an earth formation using a drilling apparatus or rig. The drill string is comprised of an outer pipe string, to the lower end of which is connected a drill bit, and an inner pipe string. The inner and outer pipe strings together define an annular passageway for communicating fluid, such as air, to the bottom of the bore hole while the inner pipe string defines a bore which serves to return the air and cutting bits to the surface.
The outer pipe string is formed by a plurality of lengths of pipe threadedly connected in end-to-end relation with each length being about ten feet in length. The outer pipe string serves to transfer the rotary and/or percussive forces to the cutting or drill bit, absorb pull up forces, which must be sufficient to overcome friction in the bore hole and the weight of both the outer and inner pipe strings when extracting the drill string. It must also carry its own weight which may be substantial depending upon the length of the drill string. Accordingly, the outer pipe string is a high strength assembly which must be designed to withstand these loads.
The inner pipe string is also formed by a plurality of serially connected lengths of pipes although not necessarily threadedly connected, each being normally about ten feet in length. The primary function of the inner pipe string is to define the two above-mentioned fluid passageways. It need not transfer rotary or percussive forces of the drill bit and, accordingly, need not meet the same high strength requirements of the outer pipe string. Thus, it may be constructed of thinner walled tube. Nevertheless, since the drill string may have considerable length, the weight imposed upon the lower lengths of inner pipe is still substantial. Thus, heretofore, it has been necessary to provide an inner pipe member having a larger thickness than would be necessary if the substantial weight of the inner pipe string were not a factor. This results in greater weight and cost of the drill string.
A further difficulty associated with drill strings is the storage, handling and assembling of pipe strings. Conventionally, the outer and inner pipe members are stored in separate racks adjacent the drilling site and the inner pipe members are inserted within the outer pipe members when the drill string is being assembled. The inner pipe member may be pre-mounted in an associated outer pipe member but this results in difficulty in handling in that the inner pipe member telescopically slides inwardly and outwardly of the outer pipe member with the not infrequent result of damage to one or both ends of the rather fragile inner pipe member.
Canadian Application Ser. No. 496,949, filed Feb. 24, 1982 issued as Canadian Patent No. 1,159,442 on Dec. 27, 1983 describes a dual-wall drill pipe assembly in which inner and outer pipe members are formed with cooperating shoulders which, when engaged, serve to locate the inner pipe member within the outer pipe member and transfer the weight of the inner pipe member, and any other axial loads applied to the inner pipe member, directly to its associated outer pipe member. As a result, the inner pipe members at the lower end of the inner drill string need not absorb any more load than the inner pipe members at the upper end of the drill string. This arrangement thereby considerably reduces the strength requirements, size and cost of the inner pipe string. There is also described releasable retaining means in the form of longitudinally extending leaf springs disposed between the inner and outer pipe members to permit preassembly of dual-walled drill pipe sections, facilitate storage and handling of drill pipe sections and assembling and dismantling of drill strings. While this arrangement has proven to be quite satisfactory, successful and superior to conventional dual-wall pipe arrangements, the leaf springs are relatively expensive to manufacture and assemble and may damage down hole tools and/or inner and outer pipe members together if a spring breaks during operation.